A kallidin-releasing enzyme with arginine ester hydrolytic activity was isolated from Bitis arietans venom by Sephadex G-75, DEAE-cellulose, and Sephadex G-100 column chromatography. This enzyme was shown to be homogeneous as demonstrated by a single band on polyacrylamide gel electrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and NH2-terminal sequence analysis. The molecular weight was determined to be 58,000 Da with an isoelectric point of 4.4. Kallidin-releasing enzyme possesses proteolytic activity demonstrated by the hydrolysis of Gly(8)-Ser(9), Ala(14)-Leu(15), Tyr(16)-Leu(17), and Phe(25)-Tyr(26) bonds of oxidized insulin B chain. This enzyme did not convert fibrinogen to fibrin, yet it did hydrolyze the A alpha, B beta, and gamma chains of fibrinogen. The enzyme was shown to cleave a kininogen analog with the release of kallidin. Arginine ester hydrolytic activity of the preparation was inhibited by diisopropyl fluorophosphate and benzamidine hydrochloride, suggesting that serine and glutamic acid or aspartic acid are involved in this activity. This protein was stable to heat and over the pH range of 3-12.